Thermoplastic elastomers are relatively new types of polymeric composition that have become commercially available in recent years. These polymers have the useful property of behaving like rubber at normal use temperatures, without the need for vulcanization. Because they are not vulcanized, they can be processed by many of the normal thermoplastic polymer procedures at elevated temperatures.
Such thermoplastic elastomers are commonly used to make shoe soles, foam products, strips incorporating chemicals and perfumes to provide a time release for such chemicals and perfumes, and pressure sensitive adhesives. Examples of thermoplastic elastomers used for such purposes are disclosed in the following U.S. Pat. Nos. 3,562,355 issued to Holden on Feb. 9, 1971; 3,562,356 issued to Nyberg & Hendricks on Feb. 9, 1971; 3,576,911 issued to Maxey on Apr. 27, 1971; 3,630,980 issued to Russell on Dec. 28, 1971; 3,769,417 issued to Van Breen on Oct. 30, 1973; 3,994,439 issued to Van Breen & Nitters on Nov. 30, 1976; 4,003,860 issued to LaClerc on Jan. 18, 1977; 4,039,629 issued to Himes & Mendoza on Aug. 2, 1977; 4,171,411 issued to Ehrenfreund on Oct. 16, 1979; 4,208,315 issued to Zweig on Jan. 17, 1980; and 4,216,131 issued to Himes & Zweig on Aug. 5, 1980.
The present invention involves thin films of thermoplastic elastomers which are blends of certain monoalkenyl arene and conjugated diene block copolymers and certain ethylene-vinyl acetate copolymers. Thermoplastic elastomeric films of this general type are disclosed in the following U.S. Pat. Nos. 3,424,649 issued to Nyberg & Hendricks on Jan. 28, 1969; 4,082,877 issued to Shadle on Apr. 4, 1978; 4,178,337 issued to Hall, Korpman & Sinker on Dec. 11, 1979; 4,199,490 issued to Kamiya, Satake, Sone & Teraoka on Apr. 22, 1980; and 4,329,309 issued to Kelly on May 11, 1982. Thermoplastic elastomers made from blends of monoalkenyl arene and conjugated diene block copolymers blended with other thermoplastic resins are disclosed in the following references: U.S. Pat. Nos. 4,173,612 issued to Kelly on Nov. 6, 1979; 4,300,562 issued to Pieniak on Nov. 17, 1981; and 4,305,990 issued to Kelly on Dec. 15, 1981; and U.K. patent application No. 2,016,262 filed on Jan. 26, 1979.
Thermoplastic elastomeric films made from elastomeric block copolymers of monoalkenyl arenes and conjugated dienes alone generally exhibit good strength and elastic properties; however, such films exhibit excessive blocking. Blocking is the tendency of a film to adhere to itself. When films made from block copolymers of monoalkenyl arenes and conjugated dienes are stored on a roll or in stacks, the films layers become difficult to unroll or unstack after a period of time due to their blocking tendency. Such blocking tendency can cause the film to tear when the film is being unwound from a roll or being unstacked.
In achieving the present invention, it was found that the addition of certain thermoplastic polymers and copolymers to the block copolymers of monoalkenyl arenes and conjugated dienes will substantially reduce the blocking tendency of the films. However, the elastic properties of the resulting film are almost always poorer than films of the block copolymer alone in that they fail to return to their original length after being stretched under certain conditions. The present invention involves the addition of particular thermoplastic copolymers to block copolymers of monoalkenyl arenes and conjugated dienes which results in films having substantially reduced blocking tendency while retaining good elastic properties.